Surviving the Storm: Cytokine Biosignature in SARS-CoV-2 Severity Prediction

Simple Summary The world has been stricken mentally, physically, and economically by the COVID-19 virus. However, while SARS-CoV-2 viral infection results in mild flu-like symptoms in most patients, a number of those infected develop severe illness. These patients require hospitalization and intensive care. The severe disease can spiral downwards with eventual severe damage to the lungs and failure of multiple organs, leading to the individual’s demise. It is necessary to identify those who are developing a severe form of illness to provide early management. Therefore, it is crucial to learn about the mechanisms and chemical mediators that lead to critical conditions in SARS-CoV-2 infection. This paper reviews studies regarding the individual chemical mediators, pathways, and means that contribute to worsening health conditions in SARS-CoV-2 infection. Abstract A significant part of the world population has been affected by the devastating SARS-CoV-2 infection. It has deleterious effects on mental and physical health and global economic conditions. Evidence suggests that the pathogenesis of SARS-CoV-2 infection may result in immunopathology such as neutrophilia, lymphopenia, decreased response of type I interferon, monocyte, and macrophage dysregulation. Even though most individuals infected with the SARS-CoV-2 virus suffer mild symptoms similar to flu, severe illness develops in some cases, including dysfunction of multiple organs. Excessive production of different inflammatory cytokines leads to a cytokine storm in COVID-19 infection. The large quantities of inflammatory cytokines trigger several inflammation pathways through tissue cell and immune cell receptors. Such mechanisms eventually lead to complications such as acute respiratory distress syndrome, intravascular coagulation, capillary leak syndrome, failure of multiple organs, and, in severe cases, death. Thus, to devise an effective management plan for SARS-CoV-2 infection, it is necessary to comprehend the start and pathways of signaling for the SARS-CoV-2 infection-induced cytokine storm. This article discusses the current findings of SARS-CoV-2 related to immunopathology, the different paths of signaling and other cytokines that result in a cytokine storm, and biomarkers that can act as early signs of warning for severe illness. A detailed understanding of the cytokine storm may aid in the development of effective means for controlling the disease’s immunopathology. In addition, noting the biomarkers and pathophysiology of severe SARS-CoV-2 infection as early warning signs can help prevent severe complications.


Introduction
Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections have resulted in the coronavirus disease 2019  pandemic. This virus has been linked to a high morbidity and mortality rate representing an extraordinary global health challenge [1]. The concerning variants of SARS-CoV-2, which have a higher transmission rate

Neutrophilia
In SARS-CoV-2 infection, neutrophilia has been observed. Under physiological conditions, neutrophils phagocytose pathogens and thus protect against disease [62]. On the other hand, when there is overactivation of neutrophils, they can cause the dissolving of connective tissue and damage other cells [63].
It was noted in a clinical study that neutrophil count was higher in severe SARS-CoV-2 infection when compared to healthy subjects [64]. In a clinical trial in Wuhan, China, the neutrophil count increased in SARS-CoV-2 infected subjects who did not survive compared to those who survived. The neutrophils continued to rise until the demise of the subjects who did not survive the infection [65]. A study of single-cell sequencing [66] and another flow cytometry study [67] noted neutrophil precursors at different stages of peripheral blood mononuclear cells development of SARS-CoV-2 patients with Acute Respiratory Distress Syndrome. A clinical study that performed RNA sequencing with peripheral blood mononuclear cells observed the presence of dysfunctional mature neutrophils expressing programmed death-ligand 1 and immature neutrophil progenitors in case of severe SARS-CoV-2 infected patients [68].
Even though the possible mechanism used by the virus to cause neutrophilia in SARS-CoV-2 infection is not understood, it has been noted in a study that glycolysis regulators, pyruvate kinase M2 level and phosphorylated pyruvate kinase M2, were increased in neutrophils of SARS-CoV-2 infected subjects who were admitted to the ICU in comparison with COVID-19 patients that were not in the ICU [69,70]. This finding suggests the reprogramming of neutrophils in severe cases of SARS-CoV-2 infection leading to overproduction and overactivation of neutrophils in severe SARS-CoV-2 infection [22].
To suppress virus particles, neutrophils liberate histone-wrapped nucleic acid structures that appear like a web known as Neutrophil Extracellular Traps (NETs) [71]. NETs' overproduction causes lung tissue injury resulting from the effects of enzymes related to NETosis such as Neutrophil elastase and Myeloperoxidase [72]. The extent of damage

Neutrophilia
In SARS-CoV-2 infection, neutrophilia has been observed. Under physiological conditions, neutrophils phagocytose pathogens and thus protect against disease [62]. On the other hand, when there is overactivation of neutrophils, they can cause the dissolving of connective tissue and damage other cells [63].
It was noted in a clinical study that neutrophil count was higher in severe SARS-CoV-2 infection when compared to healthy subjects [64]. In a clinical trial in Wuhan, China, the neutrophil count increased in SARS-CoV-2 infected subjects who did not survive compared to those who survived. The neutrophils continued to rise until the demise of the subjects who did not survive the infection [65]. A study of single-cell sequencing [66] and another flow cytometry study [67] noted neutrophil precursors at different stages of peripheral blood mononuclear cells development of SARS-CoV-2 patients with Acute Respiratory Distress Syndrome. A clinical study that performed RNA sequencing with peripheral blood mononuclear cells observed the presence of dysfunctional mature neutrophils expressing programmed death-ligand 1 and immature neutrophil progenitors in case of severe SARS-CoV-2 infected patients [68].
Even though the possible mechanism used by the virus to cause neutrophilia in SARS-CoV-2 infection is not understood, it has been noted in a study that glycolysis regulators, pyruvate kinase M2 level and phosphorylated pyruvate kinase M2, were increased in neutrophils of SARS-CoV-2 infected subjects who were admitted to the ICU in comparison with COVID-19 patients that were not in the ICU [69,70]. This finding suggests the reprogramming of neutrophils in severe cases of SARS-CoV-2 infection leading to overproduction and overactivation of neutrophils in severe SARS-CoV-2 infection [22].
To suppress virus particles, neutrophils liberate histone-wrapped nucleic acid structures that appear like a web known as Neutrophil Extracellular Traps (NETs) [71]. NETs' overproduction causes lung tissue injury resulting from the effects of enzymes related to NETosis such as Neutrophil elastase and Myeloperoxidase [72]. The extent of damage within the lung and the gravity of disease have been associated with the uncontrolled production of NET. Markers of NETosis are linked to lung inflammation [73]. 'Primed' neutrophils producing NETs have been found in subjects with ARDS with pneumonia [74]. In severe SARS-CoV-2 infection, lung inflammation is a significant complication of the respiratory system that can be life-threatening [75]. A study conducted to note the damaging role of NETs in severe SARS-CoV-2 infected patients observed a rise in NETs in plasma and aspirate from the trachea in COVID-19 patients; the neutrophils of these subjects released Vaccines 2022, 10, 614 5 of 28 significantly higher concentrations of NETs, and NETs were found in specimens of lung tissue from autopsies of infected subjects. NETs produced by neutrophils in COVID-19 patients were found to cause cell death in lung epithelium [76]. An investigation performed with SARS-CoV-2 infected patients' sera noted increased citrullinated histone, myeloperoxidase DNA, and cell-free DNA. SARS-CoV-2 patients' sera also prompted control neutrophils to release NETs [77]. NETosis triggered by a virus acts as a double-edged sword, since NETs act as a DNA web to efficiently trap viruses [78] and are also responsible for very high inflammatory processes that damage the body [79]. Symptoms of COVID-19 could thus be influenced by the link between neutrophil function in destroying virus and NETs overproduction, leading to cytokine storm [80].

Antibody-Mediated Effect of B Lymphocytes
Immunological remembrance is a mechanism to safeguard us from reinfection [96,97]. Neutralizing antibodies (NAbs) synthesized by B cells are fundamental for the immunological defense approach and lie beneath almost all viral disease control and vaccine success [98,99]. Among COVID-19 infected patients, B cells produce antibodies that target the ACE2 receptors, which impede the entry of the SARS-CoV-2 virus into cells [100,101]. Another study revealed that mild COVID-19 infection persuades vigorous antigen-specific, long-lived humoral immune memory in humans [102]. Multiple research studies revealed that convalescent COVID-19 cases possess anti-SARS-CoV-2 antibodies and had low rates of reinfection [103][104][105]. A positive association has been observed between COVID-19 disease severity and high antibodies or B cells levels [66,106,107]. The fragment crystallizable region (Fc region) is the end area of an antibody that intermingles with cell surface receptors, termed Fc receptors (FcRs), and a number of proteins of the complement system [108]. FcRs and complement interaction augment neutralization, removal of infected cells, opsonization of virions, and modulate innate and adaptive immune activity [109][110][111]. Additionally, the Fc domain of antibodies when bound with viral proteins on the surface of virus-infected cells has been reported. This interaction promoted the release of cytotoxic substances such as perforins and granzymes, subsequently killing virus and virus-infected cells [112][113][114]. A similar observation was noticed in COVID-19 cases [113,114] and other viral diseases [110,113,[115][116][117][118][119][120].

Monocyte and Macrophage Dysregulation
A significant role in the innate immune response for viral infection and inflammation is played by monocytes and macrophages. A study where single-cell sequencing of RNA was carried out noted a significant reduction in CD16 + nonclassical monocytes and CD14 + CD16 + intermediate monocytes, but a marked rise in CD14 + classical monocytes in the blood of SARS-CoV-2 infected subjects having severe symptoms. Nonclassical monocytes maintain the homeostasis of blood vessels and anti-inflammatory function, while the classical monocytes can convert to tissue macrophages leading to the inflammatory response [121]. A high expression of chemokines and cytokines by monocyte and macrophage in broncho-alveolar lavage fluid suggests inflammatory response [122]. Another study observed a shift from CD16 + to CD14 + in SARS-CoV-2 patients with ARDS and reduced CD16+ monocyte in peripheral blood in these subjects. It was also noted that gene encoding chemokines and cytokines were not upregulated in monocytes in the periphery, suggesting that cytokine storm progression in SARS-CoV-2 patients may not involve monocytes in peripheral blood [66]. The study of the broncho-alveolar lavage fluid subpopulation of leukocytes found that activation markers such as HLA-DR, CD69, CD 64, and CD16 were greater in macrophages in broncho-alveolar lavage fluid than in macrophages in the periphery [35]. These studies suggest that monocytes and macrophages in the respiratory system produce inflammatory chemokines and cytokines in severe SARS-CoV-2 infection and thus participate in the cytokine storm [22].

Response of Interferon Type 1
Clearance of the virus and regulation of innate and adaptive immunity involves the response of Interferon type 1, which is vital for combating virus infection [123]. Cells of natural immune response recognize SARS-CoV-2 RNA through NOD-like receptors, retinoic acid-inducible gene-I-like receptor, and toll-like receptor [124] with eventual activation of Interferon regulatory factor 3/7, activator protein 1, and nuclear factor kβ and promoting Interferon type 1 and inflammatory cytokines synthesis. There is the activation of tyrosine kinase 2 or Janus Kinase-signal transducer and activator of transcription 1/2 (STAT1/2) pathway and initiation of Interferon stimulated genes by Interferon type 1 [125][126][127]. However, studies suggest a marked reduction in the Interferon type 1 protective response in severe SARS-CoV-2 infection [84,128,129]. Interferon type 1 response may be inhibited through M protein, ORF6a protein, ORF3a protein, and N proteins, which are structural components of the SARS-CoV-2 virus [130][131][132][133]. Moreover, NSP1 protein found in the SARS-CoV-2 virus may cause inhibition of transcription of ISG and phosphorylation of STAT1. Inhibition of Interferon type 1 may also result from reduced plasmacytoid dendritic cells, which is a producer of Interferon type 1 during viral infection [134,135]. Plasmacytoid dendritic cells have been noted to be decreased in SARS-CoV-2 infected patients' blood and more so in those suffering severe infection [66,84]. However, a study has observed delayed Interferon type 1 response in those patients who are critically ill, since their broncho-alveolar lavage fluid contained Interferon type 1 and ISG despite the low level of systemic Interferon type 1 [127,134]. A delayed Interferon type 1 response aggravates inflammation and hinders virus clearance [135,136].

Interleukin 6
Several studies have found an increased level of Interleukin 6 in individuals suffering from severe SARS-CoV-2 infection [126,[137][138][139][140]. In Germany, a study found that Interleukin 6 levels greater than 80 pg/mL along with CRP greater than 97 mg/L showed a high degree of sensitivity and specificity for predicting respiratory failure [138]. Another retrospective study in China with 150 individuals observed a rise in the level of Interleukin 6 in severe SARS-CoV-2 subjects [137]. Interleukin 6 (released by dendritic cells, monocytes, and macrophages) is a notable activator of the pathway of JAK/STAT3, and the axis of Interleukin 6, Janus Activated Kinase, and STAT3 has a close relationship with SARS-CoV-2 severity [141,142]. JAK/STAT3 pathway activation leads to increased T helper 17 differentiation, B cells, CD + 8 T cells, neutrophil migration, and decreased Tregs formation, leading to further Interleukin 6 formation promoting inflammation [22,143].
Over-activation of the Interleukin 6-Janus Activated Kinase-STAT3 pathway may produce Interleukin 6, monocyte chemoattractant protein 1, vascular endothelial growth factor, and Interleukin 8 and decrease E Cadherin on endothelial cells [144]. Interleukin 6 may cause acute phase proteins production such as hepcidin, thrombopoietin, fibrinogen, CRP, ferritin, and complement C3 [145,146]. Studies have found monocyte chemoattractant protein 1 may be responsible for adhesion molecule formation, Vascular Smooth muscle cells proliferation, and thermogenesis [147][148][149][150]. Raised vascular endothelial growth factor and lowered E cad-Vaccines 2022, 10, 614 7 of 28 herin may result in increased permeability and leakage of the blood vessel and therefore promote dysfunction in the lung during cytokine storm in SARS-CoV-2 infection [151].
Studies have found that COVID-19 virus may cause angiotensin II expression [152,153]. Binding of the angiotensin II with angiotensin II type 1 receptor leads to JAK/STAT3 pathway activation, increasing Interleukin 6 production [154,155]. Thus, it is possible that there is an enhancement of Interleukin 6 by the SARS-CoV-2 virus and this therefore acts as a biochemical signature in severe SARS-CoV-2 infection [156].

Interleukin 7
T lymphocyte maintenance, survival, and differentiation require Interleukin 7 [157][158][159][160]. It is also essential for developing lymphoid tissue, innate lymphoid cell maintenance, and development [22]. Raised levels of Interleukin 7 in SARS-CoV-2 infection and an association between increased Interleukin 7 with the severity of disease have been observed in recent studies [19,111,161].

Interleukin 10
T helper cell 2, CD 8 + T cells, dendritic cells, B cells, Macrophages, Tregs, and natural killer cells release Interleukin 10. This Interleukin 10 has immune regulatory functions. This cytokine signaling occurs through the JAK/STAT 3 pathway. Interleukin 10 can cause activation of mast cells and promote CD8 + T cells, natural killer cells, and B cells. This cytokine can enhance Treg development and limit the immune function of macrophage and dendritic cells [162]. Although the enhanced production of Interleukin 10 attempts to subdue the hyperactive immune system, the role of Interleukin 10 is insufficient in the cytokine storm in SARS-CoV-2 infection, where there is an immense production of inflammatory cells [163].
A study observed a significantly higher level of Interleukin 10 in the plasma of SARS-CoV-2 infected patients who took admission into the ICU compared to those who were not admitted [161]. The findings of a clinical trial have noted Interleukin 10 to be a possible disease severity indicator since the study observed that serum levels of Interleukin 10 were significantly increased in patients who were critically ill than the patients suffering moderate to severe infection, with the positive correlation between Interleukin 10 and serum C Reactive Protein [164]. Another follow-up study that consisted of 71 SARS-CoV-2 infected patients and 18 control subjects found that Interleukin 10 production in the early disease stage had a significant correlation with severity of disease [165]. Interleukin 10 may cause exhaustion of T lymphocytes in the early stage of COVID-19 infection, damaging the infected patient. Thus, using a neutralizing antibody for Interleukin 10 blocking during the early stages of the COVID-19 infection can be of therapeutic use [163].

Interleukin 12
B cells, macrophage, and dendritic cells release Interleukin 12, which promotes T helper cell 1 and T helper cell 17 proliferation; enhances natural killer cells' cytotoxic function, and increases the release of Interferon γ, natural killer cells, macrophages, dendritic cells, and T helper 1 cells [166,167]. Cytokine storm aggravates several immune cells' activation. However, a recent study found no difference between plasma Interleukin 12 levels between SARS-CoV-2 infected patients and healthy individuals [161].

Interleukin 2
CD4 + T cells produce Interleukin 2, which has a fine-tuning effect on the immune response and has a vital role in natural killer cells, CD8 + cells, and CD4 + cells differentiation and expansion utilizing the Interleukin 2-Janus Activated Kinase-STAT 5 pathway of signaling [168][169][170]. A study performed in China with 54 SARS-CoV-2 infected subjects noted decreased plasma Interleukin 2 level in subjects suffering critical condition (n = 6) compared to subjects with severe illness (n = 14). Additionally, in comparison with healthy subjects, patients with critical and severe disease had significantly reduced Interleukin 2 Rα in peripheral blood mononuclear cells. In severe SARS-CoV-2 infection, the researchers suggested decreased Interleukin 2, Interleukin 2R, Janus Activated Kinase 1, and STAT5 results in lymphopenia [171].

Interleukin 17
T helper 17, ILC3, and CD8 + produce Interleukin 17 and take part in various processes of inflammation [172,173]. This cytokine's function (which may protect against infection but can also have a harmful inflammatory effect) depends on the location and type of tissue in which this cytokine is produced [174]. A study observed that Interleukin 17 played a significant part in the hyperactivation of immune cells and caused dysfunction of target organs in COVID-induced cytokine storms. The cytokine promoted neutrophil recruitment and caused inflammation infiltration, tissue remodeling, and fever [175].

Tumor Necrosis Factor α
Tumor Necrosis Factorα (TNFα) is an inflammatory cytokine produced mainly by T cells, macrophages, and monocytes [176,177]. TNFα causes activation of Nuclear Factor κβ and leads to activation of NF κβ signaling. This results in inflammatory gene expression via the TNFR1 receptor [178][179][180]. TNFα and NF κβ together enhance systemic inflammation and promote epithelial cell apoptosis. Thus, it is vital in initiating and hyper-activating immune cells in cytokine storms [22]. TNFα has been observed to be raised in severely ill COVID-19 patients [19,93,161,181]. Table 1 includes studies performed on COVID-19 infected patients showing the immune response of the body to SARS-CoV-2 virus. in severe and critical patients, there was highly impaired interferon (IFN) type I response (characterized by no IFN-β and low IFN-α production and activity), which was related with a persistent viral load in blood and hyperinflammatory response.
Inflammation was characterized by increased tumor necrosis factor-α and interleukin-6 production and signaling. infection (fever, generalised malaise, cough and shortness of breath) up to 1 week prior to admission to hospital and (2) findings of ground-glass opacity (GGO) by high-resolution CT (HRCT) of the chest as per radiology reading Elevated levels of IL6 was observed in most COVID-19 patients which was higher significantly in COVID-CS (35 vs. 96 pg/mL) confirming strong inflammation. The white blood cells, and particularly neutrophils and monocytes, were significantly increased in the COVID-CS group, suggesting innate immunity has a active role in Cytokine storm. The lymphocytes were decreased, on average half of the lower limit of normal value, indicating a functional depletion of the adaptive immunity

Interferon γ
Interferon γ (IF γ) is produced by T cells, NK cells, and macrophages. It takes part in processes of inflammation. It causes activation of T cells, NK cells, and macrophages and acts via JAK activation [184,185]. IF γ has significant involvement in the cytokine storm [186]. Various studies have found a rise in IF γ levels in SARS-CoV-2 infected subjects [161,187]. However, another study found a reduced level of IF γ in severely ill COVID-19 patients compared to those with mild infection. It may have resulted from T cells' exhaustion in severe SARS-CoV-2 infection [188].

Interleukin 1β
Interleukin 1β is produced by macrophages and promotes immune cell migration to the site of inflammation, cytokine, and adhesion factor release, and activation of the NFκβ pathway and differentiation of T helper cell 17 [189,190]. Mature IL1β is formed by cleaving of inactive IL1β. This cleaving is achieved by a complex formed by NLRP3 proteins with NLRP3 inflammasomes (caspase 1 and ASC) [22]. Studies have noted that IL1β may play an important role in the coronavirus cytokine storm [191][192][193]. Huang et al. found a rise in the level of IL1β in SARS-CoV-2 infected patients [161]. Another study conducted by Zhang et al. noted the increase in the level of IL1β in severely ill SARS-CoV-2 infected patients [194]. Activation of NLRP 3 may result from excessive reactive oxygen species production and inactive IL1β cleaving and worsening COVID-19 cytokine storm [22,[195][196][197].

Granulocyte-Macrophage-Colony-Stimulating Factor (GM-CSF) Signaling
Hematopoietic cells, endothelial cells, epithelial cells, and other cells produce the cytokine called GM-CSF [198]. GM-CSF promotes alveolar macrophage homeostasis in low levels under normal conditions [22,199]. However, during a cytokine storm, GM-CSF triggers myelopoiesis which causes myeloid cells' aggregation at the inflammation site and accelerates inflammation reactions [200]. A recent study observed a rise in GM-CSF levels in mild and severe cases of SARS-CoV-2 infection [161].

C Reactive Protein (CRP)
CRP (an acute-phase protein) increases during the early stages of inflammation [201] and has been noted to rise in patients suffering from SARS-CoV-2 infection [202][203][204]. COVID-19 virus causes macrophage complement system activation resulting in hyperinflammatory conditions, and thus may raise the CRP level [205,206]. Cytokine, like TNFα and Interleukin 6, during inflammatory stages, causes stimulation of hepatocytes to release CRP [202]. Studies have noted the rise in CRP to be significantly related to SARS-CoV-2 induced cytokine storm in severely ill subjects [206,207]. Stringer et al. reported that mortality from COVID-19 correlated with CRP cut-off value > 40 mg/L and suggested that this finding may be used as a guideline by clinicians for appropriate management and planning for care in advanced stages of the disease [207].
Xie et al., in a study carried out in 2020 with 140 hospitalized SARS-CoV-2 infected subjects having moderate to severe bronchopneumonia and needing oxygen supplementation, observed that subjects with oxygen saturation ≤90% had a median CRP level of 76.5 mg/L, which was significantly higher than subjects having oxygen saturation of >90%, that is, median CRP level 12.7 mg/L. Such findings suggest severe involvement of lung increase CRP levels [208]. A similar association between CRP levels with the early stage of COVID-19 was also noted by Wang [209].

D Dimer
D dimer is a breakdown compound of fibrin formed by fibrinolysis induced by plasmin. It is a biomarker for disorders of thrombosis [210,211]. Studies have observed an association between mortality among hospitalized COVID-19 patients and a rise in D dimer (>1.5-2.0 ug/mL) [212,213]. Yao found, in 75% of the hospitalized patients, 184 subjects out of 248 in Wuhan, China, the elevation of D dimer. The rise in D dimer was significantly associated with the severity of the disease [213]. D dimer measurement is of prognostic value for thrombosis since the thrombotic condition in arteries and veins has been found in COVID-19 patients [212,214].

Cytokine Storm
Cytokines such as TNF α, interleukin-1,2,6,7,8, Interferon γ, granulocyte-macrophage colony-stimulating factor, monocyte chemoattractant protein 1, and granulocyte colonystimulating factor are components that contribute to cytokine storm in SARS-CoV-2 infection [67,70,84,181,[215][216][217][218][219]. COVID-19 induced cytokine storm is more aggressive than other cytokine storms, as more cytokines and lymphopenia are involved, which is less common in other cytokine storms. Innate immune cells are mainly involved in SARS-CoV-2 induced cytokine storm [220]. Once the SARS-CoV-2 virus enters the cell, the viral RNA is sensed by PRRs as a pathogen-associated molecule pattern (PAMP) in the cell. There is then activation of NFκβ and IRF3/7, which leads to the formation of Interferon I (IFNI) and inflammatory cytokines [221][222][223]. SARS-CoV-2 virus removes the protective response of IFN I by several mechanisms. There is the enhancement of overwhelming inflammation and impairment of virus clearance as there are massive amounts of inflammatory cytokine production. The expected protective host immune response converts to harmful inflammation in SARS-CoV-2 infection [21].
The respiratory epithelium is infected by the SARS-CoV-2 virus, leading to chemokines such as CCL 2 and CCl3.CCl5, CXCL10, and pro-inflammatory cytokines such as Interleukins 1, 6, 8, 12, and TNF α. These chemokines and cytokines promote the aggregation of innate immune cells such as neutrophils, dendritic cells, natural killer cells, macrophages, and monocytes. There is also activation of adaptive immune cells such as CD8 + and CD4 + cells, which cause the continued formation of pro-inflammatory cytokines such as Interferon γ, Interleukin 2, and TNF α. These events cause induction of granulopoiesis and myelopoiesis and promote further damage to the lung and epithelium [22,224]. Systemic cytokine overproduction aggravates activation of macrophages as well as phagocytosis of erythrocytes, resulting in anemia [225,226]. There is also disturbance of hemostasis and coagulation, which leads to thrombosis and capillary leak syndrome [227,228]. Such a sequence of events eventually results in Acute Respiratory Distress Syndrome, failure of multiple organs, and demise ( Figure 2) [21]. Although immune regulatory cells such as Tregs produce Interleukin 10 and Tumor Growth factor β that can reduce overactive immune response, it is insufficient to antagonize the hyper inflammation state produced in cytokine storm [22,229,230].

Cytokine Storm
Cytokines such as TNF α, interleukin-1,2,6,7,8, Interferon γ, granulocyte-macrophage colony-stimulating factor, monocyte chemoattractant protein 1, and granulocyte colony-stimulating factor are components that contribute to cytokine storm in SARS-CoV-2 infection [67,70,84,181,[215][216][217][218][219]. COVID-19 induced cytokine storm is more aggressive than other cytokine storms, as more cytokines and lymphopenia are involved, which is less common in other cytokine storms. Innate immune cells are mainly involved in SARS-CoV-2 induced cytokine storm [220]. Once the SARS-CoV-2 virus enters the cell, the viral RNA is sensed by PRRs as a pathogen-associated molecule pattern (PAMP) in the cell. There is then activation of NFκβ and IRF3/7, which leads to the formation of Interferon I (IFNI) and inflammatory cytokines [221][222][223]. SARS-CoV-2 virus removes the protective response of IFN I by several mechanisms. There is the enhancement of overwhelming inflammation and impairment of virus clearance as there are massive amounts of inflammatory cytokine production. The expected protective host immune response converts to harmful inflammation in SARS-CoV-2 infection [21].
The respiratory epithelium is infected by the SARS-CoV-2 virus, leading to chemokines such as CCL 2 and CCl3.CCl5, CXCL10, and pro-inflammatory cytokines such as Interleukins 1, 6, 8, 12, and TNF α. These chemokines and cytokines promote the aggregation of innate immune cells such as neutrophils, dendritic cells, natural killer cells, macrophages, and monocytes. There is also activation of adaptive immune cells such as CD8 + and CD4 + cells, which cause the continued formation of pro-inflammatory cytokines such as Interferon γ, Interleukin 2, and TNF α. These events cause induction of granulopoiesis and myelopoiesis and promote further damage to the lung and epithelium [22,224]. Systemic cytokine overproduction aggravates activation of macrophages as well as phagocytosis of erythrocytes, resulting in anemia [225,226]. There is also disturbance of hemostasis and coagulation, which leads to thrombosis and capillary leak syndrome [227,228]. Such a sequence of events eventually results in Acute Respiratory Distress Syndrome, failure of multiple organs, and demise ( Figure 2) [21]. Although immune regulatory cells such as Tregs produce Interleukin 10 and Tumor Growth factor β that can reduce overactive immune response, it is insufficient to antagonize the hyper inflammation state produced in cytokine storm [22,229,230].  A study advised the inclusion of a ratio of peripheral blood oxygen saturation to a fraction of inspired oxygen, cytokine to chemokine ratio, neutrophil to lymphocyte ratio, C reactive protein, and ferritin for diagnosis of cytokine storm in SARS-CoV-2 infection [231]. In another study, certain criteria have been suggested to diagnose SARS-CoV-2 induced cytokine storm. The criteria include (a) Lymphocyte < 10.2%, absolute count of neutrophil > 11.4 × 10 3 /mL, albumin < 2.87 mg/mL, (b) D-dimer > 4930 ng/mL, troponin > 1.09 ng/mL, lactate dehydrogenase > 416 U/L, alanine aminotransferase > 60 IU/L, aspartate aminotransferase > 87 IU/L, and (c) blood urea nitrogen:creatinine ratio > 29, potassium > 4.9 mmol/L, chloride > 106 mmol/L, and anion gap < 6.8 mmol/L. Also included are C-reactive protein > 4.6 mg/dL and ferritin > 250 ng/mL 182 [232]. Hyperinflammation screening in the laboratory and performing an HS score in severe COVID-19 illness have been proposed in a study for identification of SARS-CoV-2 induced cytokine storm [19].

Predictive Factors and High-Risk Case of Cytokine Strom
Multiple studies reported that elevated liver enzymes, LDH, D-dimers, and troponin I indicate massive cell death, which leads to significant systemic tissue damage, particularly in the liver, the cardiovascular system, and kidney, suggestive of the COVID-19 cytokine storm (COVID-CS) [233][234][235][236]. Another study reported that five markers (Alanine aminotransferase (ALT), aspartate aminotransferase (AST), D-dimers, lactate dehydrogenase (LDH), and troponin I) of tissue injury were more significantly raised among patients with COVID-CS than among other SARS-CoV-2 cases [182,[237][238][239]. COVID-19 patients' ALT and AST levels exceed three times the upper limit, indicating liver damage [240]. Exalted D-dimer status is a forecaster for severe SARS-CoV2-infection and indicates thromboembolic disorders. Nevertheless, it is also a direct result of acute lung injury, as observed in COVID-19 pneumonia [241][242][243]. It has been reported that elevated LDH levels among COVID-19 cases suggest acute cell death and severe cases [244,245]. It has been revealed that, among COVID-19 cases, the LDH level rising~6 and~16-times in odds denotes developing severe disease and mortality, respectively [244]. Cardiac troponin is considered a marker of myocardial or heart muscle injury, and is thereby well-known as the first emblem of heart damage in blood. Several earlier cases of COVID-19 in Wuhan, China were hospitalized for respiratory issues and had a high level of troponin [246]. Raised troponin levels are frequently found among SARS-CoV-2 cases and suggestively indicate deadly consequences [247,248]. One more systematic meta-analysis analyzing 40 papers reported that interleukin-6, ferritin, leukocytes, neutrophils, lymphocytes, platelets, C-Reactive Protein, procalcitonin, LDH, AST, creatinine, and D-dimer are authoritative biomarkers of COVID-CS. Higher-up levels of interleukin-6 and hyperferritinemia need to be considered as a warning of fatal clinical outcomes, including death [249].
The factors that promote the risk of COVID-CS are male, >40 years, the positive test result for replicative SARS-CoV-2 RNA, absolute lymphocyte count (<0.72 × 109/L), and dynamics in the National Early Warning Score 2 (NEWS2) score, as well as LDH (>23 pg/mL), D-dimer, ferritin, serum CRP (>50 mg/L), and IL-6 levels [250]. One more meta-analysis verified that the inflammatory biomarkers, notably white blood count (WBC), absolute lymphocyte count (ALC), absolute neutrophil count (ANC), platelet count (PLT), C-reactive protein (CRP), ferritin, D-dimer, LDH, fibrinogen, and erythrocyte sedimentation rate (ESR), were correlated with the prognosis and severity among COVID-19 pediatric cases presenting with the multisystem inflammatory syndrome (MIS) [251]. One Mexican study reported that the mortality rate was 36.84% of the studied sample. The patients who passed away were 59.71 ± 13.83 years, statistically significantly higher than those who survived (43.29 ± 11.80 years). Additionally, serum levels of Interleukin 6 (IL-6) were significantly higher in patients with fatal outcomes. Furthermore, a correlation was detected between IL-6 levels with lymphocyte count, LDH, CRP, and procalcitonin (PCT) [252]. One more study reported that during admission of COVID-19 patients, the levels of SpO2, lymphocyte, CRP, PCT, and LDH often determine the prognosis of severity and clinical outcome. Additionally, systematic inflammation with cardiac complications frequently regulates fatality in severe SARS-CoV-2 apart from acute respiratory distress syndrome [253]. Another study determined that IL-6 and CRP levels serve as robust interpreters for those patients who require ventilator support [136]. One more study reported that the median levels of IL-6 were <1.5 pg/mL {(Interquartile range (IQR) < 1.50-2.15]), 1.85 pg/mL (IQR < 1.50-5.21), and 21.55 pg/mL (IQR 6.47-94.66) for the common, severe, and critical SAR-CoV-2 groups, respectively (p < 0.001). The follow-up kinetics revealed serum IL-6 was high in critical patients, even when cured. An IL-6 concentration higher than 37.65 pg/mL was predictive of in-hospital death Area under Curve (AUC) 0.97 (95% CI 0.95-0.99), p < 0.001 with a sensitivity of 91.7% and a specificity of 95.7% [254]. Multiple studies report the poor clinical outcome of COVID-19 patients frequently observed among the elderly population and in the presence of comorbidities which include angiotensin-converting enzyme-2 polymorphism, cancer, cancer chemotherapy, chronic kidney disease, thyroid disorder, diabetes, CVD, hypertension, oxidative stress, vitamin deficiency, or hematological disorders [255][256][257][258]. Another study reported that at least 50% of patients who develop COVID-CS had been elderly and suffering from hypertension or diabetes mellitus or both [259]. The aging process and the mentioned comorbidities promote oxidative stress and endothelial dysfunction [260,261]. These two comorbidities frequently act as determinant factors of SAR-CoV-2 disease prognosis, severity and even death [259].

The Current Therapeutic Options for Cytokine Storm and Time to Intervene
The recent clinical data exhibit that tocilizumab (TCZ), an anti-interleukin-6, is reasonably effective with a low potential to cause adverse drug effects (ADRs) and, in doing, results in a lower death rate among SARS-CoV-2 infected hospitalized patients [297][298][299]. Another study reported that TCZ's pharmacotherapeutic benefit was more observed when administered along with corticosteroids (CS) and when this combination was given within the first 10 days of symptoms [300,301], whereas one more study reported that TCZ as monotherapy showed substantial benefit among patients who had a comparatively low level of the ratio of ferritin/CRP, thereby quickly decreasing the CRP, IL-6, IFN-γ, IP-10, and MCP-1 levels. Nevertheless, the high proportion of ferritin/CRP is related to the swift deterioration of respiratory function. This study suggested that for patients with a high ratio of ferritin/CRP, TCZ in combination with glucocorticoids is a better therapeutic option [302].
SARS-CoV-2 infection dysregulated cytokine release, or COVID-CS, is the major pathogenesis and remains the primary reason for multiple organ failure and death [303,304]. The role of glucocorticoids comes to the forefront, especially in managing the hyperinflammatory state of SARS-CoV-2, otherwise called COVID-CS, because of its dominant role in immunosuppression [305]. Nevertheless, the timing of CS administration remains very critical in SARS-CoV-2 infection [305,306]. Additionally, proper CS and dose adjustment selection remains ambiguous [307]. If CS administered in COVID-CS achieves a beneficial effect, it can be detrimental if provided too early in COVID-19 cases [305]. After that, glucocorticoids should be justified based on two diverse pathophysiological phases of the SAR-CoV-2 infection: (1) in the initial stage, administration of pharmacological doses of CS might essentially upsurge the plasma viral load because of immunosuppression, and (2) in the late phase (hyperinflammatory COVID-CS stage), CS pharmacodynamic properties will suppress hyperinflammation and alleviate the COVID-19-induced cytokine storm [308]. Furthermore, adrenal cortical steroid, programmed cell death protein (PD)-1/PD-L1 checkpoint inhibition, cytokine-adsorption devices, intravenous immunoglobulin, monoclonal antibodies, low molecular weight heparin, and antimalarial agents have been reported as potentially beneficial and dependable therapeutic options for the management of COVID-CS [297,[309][310][311][312][313][314][315][316][317][318][319][320]. Plausibly, these therapeutic options possibly will work and safeguard COVID-CS patients. Nevertheless, the probability of successful treatment greatly depends on selecting the correct patients, correct medication, appropriate dose, and correct time to intervene [321][322][323].

Limitations of this Paper
The following were some limitations of the study: This study is a narrative review, so a meta-analysis was not conducted. Studies in languages other than English could not be included. Articles that need to be accessed through institutional access could not be accessed.

Conclusions
Studies of clinical and basic research have noted that the characteristics of SARS-CoV-2 induced cytokine storm and have facilitated our knowledge of the pathophysiology of SARS-CoV-2 infection and cytokine storm. There is the involvement of many more cytokines in the COVID-19 infection-related cytokine storm than in the case of other conditions. The cytokine storm, in this case, is far more damaging and inflammation is much more severe. It has been noted that the innate immune response produces these inflammatory cytokines. The role of each cytokine in the development of cytokine storms is not yet completely understood. However, there is undoubtedly a close association between pathogenic alterations in SARS-CoV-2 infection and the SARS-CoV-2 induced cytokine storm. Induction of prolonged and excessive cytokines by SARS-CoV-2 may result in lung damage and failure of multiple organs. Studies have been performed on peripheral blood chemokines and cytokines, and the immunopathological damage due to the SARS-CoV-2-induced cytokine storm may be the reason behind severe complications and death in COVID-19 infection. There is a need for extensive molecular and epidemiological studies to understand the specific inflammatory signaling pathways and develop effective means for curtailing the virus from further spreading globally.

Recommendation
We recommend extensive molecular and epidemiological studies should be carried out in the future to understand the inflammatory mediators' roles further. Treatment strategies targeting the signaling pathways and cytokines of inflammation should be developed. Drugs that target cytokines play a vital role in the cytokine storm, and cells that produce cytokines, such as macrophages, monocytes, dendritic cells, and natural killer cells, need to be developed. Studying the immune-regulatory system and recognizing anti-inflammatory cytokines such as Interleukin 37's effects on inhibiting inflammation can help develop therapeutics to improve patient condition [232,324,325]. In addition, specific treatment plans need to be prepared, as the extent of the cytokine storm and treatment may vary with comorbidity, age, and immunity status. Early detection of severe SARS-CoV-2 infection biomarkers may help prevent multiorgan damage and death. Therefore, the development of precise treatment plans and increasing knowledge regarding the pathogenesis of COVID-19 infection is consequently of great importance.

Article Highlights
A significant part of the world population has been affected by the devastating SARS-CoV-2 infection. Evidence suggests that the pathogenesis of SARS-CoV-2 infection may result in immunopathology such as neutrophilia, lymphopenia, decreased response of type I interferon, monocyte, and macrophage dysregulation. Excessive production of different inflammatory cytokines leads to cytokine storm in COVID-19 infection. The large quantities of inflammatory cytokines trigger several inflammation pathways through tissue cell and immune cell receptors. Such mechanisms eventually lead to complications such as acute respiratory distress syndrome, intravascular coagulation, capillary leak syndrome, failure of multiple organs, and, in severe cases, death. Treatment strategies targeting the signaling pathways and cytokines of inflammation should be developed. Early detection of severe SARS-CoV-2 infection biomarkers may help prevent multiorgan damage and death. Therefore, developing precise treatment plans and increasing knowledge regarding the pathogenesis of COVID-19 infection is of great importance.
Author Contributions: R.A. and M.H. made a significant contribution to the work reported, whether that is in the conception, study design, execution, acquisition of data, analysis, and interpretation, or in all these areas; took part in drafting, revising, or critically reviewing the article; have agreed on the journal to which the article has been submitted, and decided to be accountable for all aspects of the work. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Acknowledgments:
The authors express gratitude to Naufela Nafisa Ahmad, Master of Arts in English Language (Linguistics), Jalan Wangsa Delima 7, Wangsa Maju, 53300 Kuala Lumpur, Malaysia, for revising and providing her expert opinion about the quality of the English language of this article. The authors also show gratitude to Faiza Binte Mozammel, Photographer and Editor, 7/16/1 south Mugdapara Dhaka, Bangladesh, for her kind effort and time regarding image development and editing.

Conflicts of Interest:
The authors declare no conflict of interest. The author declares that they do not have any financial involvement or affiliations with any organization, association, or entity directly or indirectly involved with the subject matter or materials presented in this article. This includes honoraria, expert testimony, employment, ownership of stocks or options, patents or grants received or pending, or royalties.